Color Printing Replica Color Conversion Principle

Device-independent color spaces are mainly used for color modules and for conversion between RGB and CMYK modules.

Each color display has its own color gamut due to different RGB phosphors, even if it was manufactured by the same manufacturer in the same year. The same applies to printing presses and CMYK toners, usually with more color gamuts than most displays. The limitation is that the colors produced by the two different formats of RGB and CMYK will differ due to the difference between the device and the device. This is the characteristic of the color space to which the device belongs.

Some color spaces allow colors to be expressed in a device-independent manner. The color does not depend on any particular device, but instead is a true color representation perceived by the human eye. These color representations are called device-independent color spaces and were developed in 1931 by the International Standards Committee (CIE) and are therefore called CIE-based color spaces.

The goal of CIE is to establish a standard system of repeatable color communication for paint, ink, dye, and other pigment manufacturers. The most important function of these standards is to provide the overall structure of color matching. Device independent color space is used for color data conversion from one device-specific color space to another device-specific color space, and those representing the entire visible color range are conversion spaces. This means that any color selected on the display is the color gamut of this neutral color space.

CIE L*a*b* is a three-degree color space based on human color sense. It is the most widely used color space of CIE. The L*a*b* color space is based on the fact that one color cannot be green and red at the same time, nor can it be The theory is blue and yellow, and as a result, a single shade can be used to describe the red/green and yellow/blue properties. The CIE L*a*b* space represents the color associated with the reference white point, which is the white light's specific meaning, usually based on the white light the device can produce. The CIE color space constitutes an independent color base for devices used for color management.

Although colors can be copied in the way that comes to mind, there are only two basic methods - additive and subtractive, both commonly known as trichromatic methods, which are based on the different principles of the three primary colors to create full color. Understanding these two systems will understand the principle of many color reproduction operations in printing. It is the basis for understanding the tone reproduction, gray balance, and color correction of halftone color reproduction, and obtaining proper contrast, color balance, and hue. It's decisive.

Printing is based on the subtractive method and is usually printed on white, near white paper, or any other white object. Since the surface of the "white" paper reflects all the light waves in equal amounts, white appears. When the color is reproduced, we It will intuitively assume that the color is in the paper. In fact, after printing on the surface of the paper with clear inks (cyan, magenta, and yellow), the red, green, and blue light waves are filtered out after multiple bonds, and the paper itself does not change color. Considering the transparent four-color inks, such as the red, green, and blue wavelengths we see in our eyes, rather than those that are combined on paper to reproduce the colors, we have found that by combining the two ink light waves and subtracting the other ink light wave.

Theoretically, combining all three color inks can avoid all reflected light waves and produce black color. Even the best four color inks will not uniformly absorb light waves. When combined with cyan, magenta and yellow ink, the combined three-color ink absorbs. The wavelengths are not uniform and the red part of the spectrum reflects more. The result is a brownish tone rather than black. In order to make up for this disadvantage in printing, black ink is used in the color printing process.

Paper has a significant effect on color reproductions. Paper reflects non-absorbed light waves to the viewer, and coated paper has a strong reflective surface that produces a wider color range than uncoated paper. Rough non-coated paper The surface will spread the light, reducing the amount of reflection on the viewer, and therefore feel darker.

The issue that has been explored since the previous color reproduction. Prior to the 1970s, most color separations were based on red, green, and blue color filters placed in front of a plate-making camera or magnifying lens. Skilled technicians used their own experience to obtain a good color reproduction. From the manuscript, multiple procedures are used to create color correction films, color separation films, and mesh, and then the dots are etched to obtain the desired halftone mesh. Later, some manufacturers used the contact exposure method to change the size of outlets. With many adjustments, it took a long time, but it may not be able to obtain the desired color.

After the 1970s, it was the era of high-order electronic or laser scanners that used innumerable knobs and buttons to make color adjustments. Although these expensive analog scanners lack computer memory and digital devices to store images, later models convert the RGB digital color signals to CMYK color separations by the computer, the original is mounted on a transparent cylinder, and the other cylinder is The dichroic film is installed. The scanner is calibrated by experts who are trained in advanced technology and make the best input and output settings. However, re-sweeping is still quite common. Especially for prints with special colors, if the colors do not match, the adjustment of the scanner or the knowledge of the operator is usually not a limiting factor, but is affected by the quality of ink and paper.

With the addition of sophisticated color controls on the scanner, printing has changed from process to pre-press, post-press or post-press, from macroscopic adjustments to color controls via several concentration meters, tube charts, ink and paper tests. Technology.

In the 1980s, advances in minicomputers, memory, and storage devices enabled color electronic prepress systems (CEPS) for storage, display, color correction, and image matching. For example, Scitex Response and Hell Chromacom's multi-million-thousand-million-dollar system can obtain high-quality color reproductions, but it also takes a long learning time.

In the 1990s, a powerful desktop prepress system appeared. Innovative page description output (postscript), image platforms such as Apple, Microsoft, and publishing software such as Adobe Photoshop, Quark Xpress can use desktop hardware to make high-end color quality, and its ease of use, low-cost combination system quickly replaced Expensive stand-alone system.

Scanning and color separation, once the need for highly trained experts, became popular, anyone including photographers, designers, typeetters, and computer operators could operate a computer, scanner, and printer to accomplish separation. Full operation. Desktop computers use open systems and theoretically support unlimited color computer peripheral devices. Today, many of these color publishing products are able to consistently operate, but they still have to face the same problems and how to make accurate colors.

Source: HC360 HC Printing Industry Channel